The dewaxing of mineral oils is known and is performed on a commercial scale by processes known as solvent dewaxing and urea dewaxing. In these two major processes, of which a great number of different embodiments has been described, first a solution of the mineral oil to be dewaxed in an organic solvent or a mixture of organic solvents is prepared, as, for example, explained in the book entitled "Mineralole . . ." by C. Zerbe, 2nd edition (1969), part I, pp. 472 and 482-83. (To eliminate the repeated use of "organic solvent or a mixture of organic solvents", the term "solvent" will be used hereinafter and shall be taken to include a mixture of solvents unless specified to the contrary.)
In the solvent dewaxing process, this solution of mineral oil and solvent is cooled down, whereby paraffins are depositing as crystals. In the urea dewaxing process, the solution is contacted with urea or a urea solution and the n-paraffin/urea insertion compounds or adducts, depositing as crystals, are separated. These adducts are then decomposed to recover an n-paraffin product and urea or urea solution. In both processes a solution of the dewaxed mineral oil in the respective solvent is obtained. Since both processes are operated continuously, it is necessary to separate the solvent from the dewaxed mineral oil and to recover the solvent which is normally done in multistage flash evaporator stations of which at least the last stage is a steam stripper.
The known dewaxing processes are disadvantageous insofar as the recovered solvent may contain great quantities of water coming partly from the feed oil to be dewaxed and partly from the strippers in the evaporator station. Due to the continuous recycling of the recovered solvent this water content is constantly increasing up to an equilibrium value and may involve considerable difficulties:
1. In the cooling stage of the solvent dewaxing process, the operating and cooling performances of the heat exchangers-- which are mostly scraped wall exchangers-- are greatly impaired by the formation of ice. They frequently need to be turned off, thawed and cleaned, as explained, for instance, in U.S. Pat. Nos. 2,478,456, column 1, and 2,949,419, column 6, line 42.
2. In the filtering stage of this process, ice crystals form when water-containing solvents are used for washing the slack wax cake at the usual operating temperatures of about -20.degree. to -40.degree. C. and said ice crystals clog the pipes, equipment, nozzles and filtering cloth (cf., e.g., German Pat. No. 1,545,279, column 1, line 62 to column 2, line 10).
3. Dissolving capacity and selectivity of the solvent may be unfavorably changed which reduces quality and yield of the products (cf., U.S. Pat. No. 3,105,809, column 6, lines 59 to 63).
4. Halogenated hydrocarbons and other solvents hydrolytically or thermally unstable may split off corrosive substances at the elevated temperatures prevailing in the flash evaporator station. These corrosive substances corrode the flash evaporation equipment and make it necessary to use expensive non-corrosive materials (cf. U.S. Pat. No. 3,105,809, column 6, line 60).
5. In urea dewaxing with crystalline urea, water contained in the mineral oil/solvent mixture changes the selectivity of adduct formation as well as the form and consistency of the adducts. If a fine-powdery adduct is to be obtained, a relatively low water content may not be exceeded.
There are known processes which are directed to reducing the water content of the solvent used in solvent dewaxing units. In U.S. Pat. No. 2,478,456 or German Pat. No. 909,386 a method of chemical drying is described in which the drying agent has to be either regenerated at high cost or rejected. The process disclosed in U.S. Pat. No. 2,478,456 only changes the form of the ice developing in the cooling stage of a solvent dewaxing process, so that no solid ice is deposited at the walls of the cooler. This requires, however, the introduction of a substance foreign to the system but does not prevent such undesirable side effects as corrosion and negative effect on the dissolving capacity.
Another commonly used process dries the recovered water-containing solvent condensate by azeotropic distillation, optionally, with prior partial extraction of the solvent by means of the dewaxed mineral oil filtrate or molten wax (cf., U.S. Pat. Nos. 2,949,419 and 3,130,143). However, the different embodiments of this known drying process are rather expensive, since several drying and distillation columns, additional extractors, several separators, and the like are required. Besides, they have the disadvantage that water entrained with the feed oil is constantly introduced into the dewaxing unit and only removed in the drying stage of the solvent recovering unit. Thus, the above-mentioned disturbances continue to exist in the majority of the various previous stages of the dewaxing process.
This disadvantage is also involved in the process disclosed in German Auslegeschrift 1,545,279. The known process is directed to the recovery of chlorinated hydrocarbon solvents from the dewaxed mineral oil filtrate and the separated crude n-paraffin or slack wax obtained in solvent dewaxing. The solvent vapors from the first oil filtrate evaporator are condensed in two steps and the water-free and wax-free solvent portion liquefied in a first condenser is passed to a main solvent collector, while the non-condensed water-containing solvent portion is liquefied in a second condenser and, after water separation, is introduced as part of the recycled solvent, into a rectifying column into whose lower part the water-containing and wax-containing solvent vapors from the first crude paraffin evaporator are introduced. The water-containing overhead product from the rectifying column is introduced into the second condenser, and the solvent vapors from the last oil filtrate flash evaporator and the last crude paraffin flash evaporator are passed, after condensation and separation of water, as a second part of the recycled substances into the rectifying column, while the water-free n-paraffin-containing bottoms product from the rectifying column is added, via a second solvent collector, to the feed oil to dilute the latter.
It is the main object of the invention to provide a process for dewaxing mineral oils which is free of the above described disadvantages and enables working under essentially water-free conditions in a simple manner.